One of the best out there would be Genesis , developed by New Millenium. If you are looking to integrate an accounting package into an existing Filemaker solution, then Genesis is your best bet, as it's been built to integrate.
The accounting module from KiBiz Systems is very rich and designed to be bolted on to existing soltuions: http://kibizsystems.com/products/product_detail.php?name=KiBiz_Accounting
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